Invasive cardiac infections are a recognized yet poorly understood facet of Listeria monocytogenes pathogenesis. Preliminary examinations of selected L. monocytogenes isolates indicates that a sub-population of strains exhibit an enhanced ability to invade and infect cardiac tissue. It is hypothesized that these cardioinvasive strains have developed novel mechanisms and/or express novel or altered factors that enable them to replicate within cardiac tissue. The goal of this project will be to elucidate the mechanism(s) by which cardioinvasive isolates of L. monocytogenes target the heart for bacterial replication. The aims of this project are designed to elucidate the course and pathology of cardiac infection by L. monocytogenes as well as to identify bacterial factors responsible for increased cardio-tropism. Mouse models of infection will be used to define the time course and tissue distribution of cardioinvasive strains of L. monocytogenes. In vivo infections in mice will be monitored using bioluminescence imaging of L. monocytogenes strains containing lux reporter fusions, and subsequent histological examination of target tissues will be used to further elucidate pathogenesis and host inflammatory response. Comparative genomic analysis of cardioinvasive and noncardioinvasive strains will be used in combination with genetic approaches to identify bacterial factors that contribute to bacterial invasion and replication within cardiac cells. The use of transposon insertion mutagenesis in a highly cardioinvasive strain and the generation of bacterial insertion libraries will be done to enable the identification of bacterial factors that contribute to invasion and replication within cardiac cells. The libraries will be screened for mutants that lack the ability to invade and replicate within cardiac cell lines. Genetic analyses of these mutants will be used to elucidate virulence mechanisms responsible for the cardiotropism of cardioinvasive strains. The information gained from these studies may ultimately enable the identification of cardiotropic strains early in the patient infection as well as in food-borne outbreaks, so as to increase the likelihood of successful diagnosis and treatment of at-risk individuals. PUBLIC HEALTH RELEVANCE: Cardiac infections represent a significant but poorly characterized facet of Listeria monocytogenes pathogenesis. Sub-populations of clinical isolates of L. monocytogenes exhibit an enhanced ability to colonize the hearts of infected animals, and our goal is to elucidate the mechanisms responsible for this mode of infection. The outcomes of this project may lead to methods of rapidly identifying such strains early in the course of an infection or outbreak, potentially leading to better treatment methods and outcomes.